Wk 2 Flashcards
what are the 4 errors
systematic
random
instrumental
personal
describe systematic error
Caused by known factors, predictable, and can be corrected. Examples: calibration errors
describe random error
Unpredictable errors due to environmental variations or human limitations. Minimized through repeated measurements
describe instrumental error
Arise due to imperfections in survey instruments. Regular calibration and maintenance help reduce them
describe personal error
Caused by observer mistakes such as misreading or incorrect levelling. Training improves accuracy
define accuracy
what error is it effected by?
Accuracy is the “closeness of the measurements to a true value.” Measurements with a low level of systematic error have a high accuracy. Measurements with a high level of systematic error are “biased”
Systematic errors impact ACCURACY of a measurement
define precision
what error is it effected by?
Precision is the “closeness of repeated measurements of the same thing.” Precise measurements will have low spread relative to their measure of central tendency. Measurements with a low level of random error have a high precision. Measurements with a high level of random error have low precision
Random errors impact PRECISION of a measurement.
When measuring what 6 ways can we use to know if our answers are correct?
❑ Repeat the measurement to check for consistency
❑ Use a different technology or method for validation
❑ Compare results with known standards or reference values
❑ Minimise errors by improving measurement techniques
❑ Cross-check with another observer or instrument
❑ Compare measurements with established control points
define control network
- Provides precise positioning for mapping and surveying
what is primary control network usually?
Primary control network is usually geodetic (large-scale projects)
define ellipsoid
mathematical model of the Earth’s shape, resembling a squashed sphere, used as a reference surface
What is OSGB36?
historical geodetic datum that underpins Britain’s triangulation network
-been replaced now
- using triangulation and geodetic leveling and defines absolute positions in relation to the Airy 1830 ellipsoid
what is ETRS89?
Modern geodetic networks like ETRS89 use GNSS/GPS
-higher precision
- based on the GRS80 ellipsoid
what are secondary control networks?
can be Geodetic or localised) used for urban & infrastructure projects
- If derived directly from primary geodetic points, it retains geodetic characteristics
define northing, easting
❑ Northing: The coordinate representing the distance north or south from a reference point
❑ Easting: The coordinate representing the distance east or west from a reference point
reference system used in horizontal control
define horizontal control
Methods to Establish Horizontal Control?
typically use a local reference system based on a network of fixed points that define positions in a planar coordinate system. It provides a reference for locating features and setting out engineering projects
- Coordinates are expressed as Northing and Easting for plane surveys
- Latitude and Longitude for geodetic surveys
Methods:
Triangulation - Uses measured angles and at least one known baseline to calculate positions
Traversing - A sequence of measured distances and angles to extend control networks
GPS/GNSS Surveys - Provides high-accuracy horizontal positioning using satellite signals
define vertical control
Methods to Establish Vertical Control
is a system of fixed benchmarks that provide elevation reference points for determining heights
Methods:
❑ Differential Levelling - Uses a levelling instrument and staff to measure height differences between points
❑ Trigonometric Levelling - Uses angles and distances to calculate elevations (common in mountainous terrain)
❑ GNSS Heighting - Provides ellipsoidal heights, converted to orthometric heights using geoid models
main diff between horizontal and vertical control
Horizontal control ensures precise planar positioning, while vertical control provides elevation references
what can EDM be used for?
measure distances up to 100km but with corrections for the earth curvature along with temperature and
pressure differences.
Electronic Distance Measurement
What is total station?
instrument works on a tripod and combines electronic distance measurement (EDM) and angle measurement for accurate positioning
- can measures distances as well as angles (Stores measurements
digitally, reducing manual errors and allowing for direct CAD integration) - Data Collection & Automation
- Remote Sensing & GPS Integration (georeferencing and large-scale surveys)
Total Station not only measures distances but also determines horizontal and vertical angles using
electronic goniometers (angle measurement devices)
what is total station used for?
used in land surveying, construction layout, topographic mapping, and deformation monitoring
how does total station work?
Light Pulse (Time-of-Flight Measurement)
- Uses a laser pulse sent from the instrument
- Measures the time it takes for the light to travel to the target and return
- Distance is calculated using the speed of light
OR
Reflector based phase shift method to determine distances and angles
what’s the allowable misclosure eqn (transversing)
eqn for allowable misclosure
eqn for the sum of interior angles
n= no. of observed angles
1) calculate what’s required to make the angle = 540°
(=correction)
2) divide the correction by how many observed angles there are
3) find the adjusted angles= add the correction to the observed angles
Define levelling
measures vertical distances relative to a horizontal reference plane
always carried out relative to a known datum (benchmark or reference point)
Purpose of levelling
❑ Essential for infrastructure projects (e.g. roads, highways, urban drainage)
❑ Ensures proper water drainage, preventing accumulation, flooding, and erosion
❑ Used in stormwater and sewer system planning to
maintain flow efficiency
What is the modern benchmark tool used?
Current systems link to WGS84 (World Geodetic System 1984) and Newlyn for accurate global positioning
What’s the history of heighting methods
❑ Since 1989, OBM (Ordnance Survey Marks) are no longer maintained
❑ GNSS (Global Navigation Satellite Systems) is now used for
height determination, ensuring higher accuracy and global compatibility
how does GNSS work
combines multiple satellite systems for precise positioning:
GPS (USA),
GLONASS (Russia),
Galileo (EU),
BeiDou (China)
Height of the Plane of Collimation
how does automatic level work? (4 components)
- Self-Levelling Mechanism :
Uses an internal compensator system to automatically adjust the line of sight and reduce errors
- Telescope & Crosshairs:
The surveyor looks through the optical telescope to read height measurements on a levelling staff
- Levelling Setup:
Mounted on a tripod, adjusted with levelling screws, and fine-tuned using a bubble vial
- Measurement Process:
Takes backsight (BS) and foresight (FS) readings to calculate elevation differences
What equipment is required for levelling?
level and tripod, staff, crowfoot, bubble
how do you check for collimation error?
perform a Two Peg Test
what’s the allowable misclosure onsite
- where 𝑛 is the number of instrument set ups
- m is set dependent on-site conditions.
A value of 𝑚 often used is 5mm, and for earthworks, it may be 30mm. 𝑚 may be set in the contract documents
define triangulation
Triangulation - Uses measured angles and at least one known baseline to calculate positions
define traversing
Traversing - A sequence of measured distances and angles to extend control network
- its actually levelling
